|By Steve Holland

PALM BEACH, Fla. (Reuters) - U.S. President Donald Trump said on Wednesday he hoped an unprecedented summit with North Korean leader Kim Jong Un would be successful after a recent visit to Pyongyang by CIA Director Mike Pompeo, but warned he would call it off if he did not think it would produce results.

Trump told a joint news conference with Japanese Prime Minister Shinzo Abe that his campaign of "maximum pressure" on North Korea would continue until Pyongyang gave up its nuclear weapons.

He also said Washington was negotiating for the release of three Americans held by North Korea and there was "a good chance of doing it." He did not answer a reporter's question as to whether that would be a condition for going ahead with the summit.

“I hope to have a very successful meeting (with Kim),” Trump said in Palm Beach, Florida.

“If I think that it’s a meeting that is not going to be fruitful, we’re not going to go," he added. If the meeting when I’m there is not fruitful, I will respectfully leave the meeting.”

Trump said earlier that Pompeo, one of his most trusted advisers and his pick to be the next U.S. secretary of state, formed a "good relationship" with Kim when he became the first U.S. official known to have met the North Korean leader.

U.S. officials said Pompeo met Kim when he visited Pyongyang over the Easter weekend, which ran from March 31 to April 2, to lay the groundwork for the planned summit, in which Trump hopes to persuade North Korea to abandon development of nuclear missiles capable of hitting the United States.

"Mike Pompeo met with Kim Jong Un in North Korea," Trump tweeted earlier. "Meeting went very smoothly and a good relationship was formed. Details of Summit are being worked out now. Denuclearization will be a great thing for World, but also for North Korea!"

A senior administration official said Pompeo brought up the case of the three American prisoners with Kim in North Korea and that the United States was hopeful for their release.

GOODWILL

Pompeo's visit provided the strongest sign yet of Trump's willingness to become the first serving U.S. president to meet a North Korean leader.

Trump said on Tuesday he believed there was a lot of goodwill in the diplomatic push, which he has said could take place in late May or early June.

U.S. officials said the visit by Pompeo was arranged by South Korean intelligence chief Suh Hoon with his North Korean counterpart, Kim Yong Chol, and was intended to assess whether Kim was prepared to hold serious talks about giving up his nuclear weapons.

They said Pompeo's conversations in Pyongyang fueled Trump’s belief that productive negotiations were possible, but far from guaranteed. They said no site had yet been chosen for a summit.

Pompeo flew to North Korea from a U.S. Air Force base in Osan, south of Seoul, an official with South Korea's Defense Ministry said.

Pompeo told his Senate confirmation hearing last week for secretary of state that he was optimistic a course could be set at a Trump-Kim summit for a diplomatic outcome with North Korea, but added that no one was under any illusion that a comprehensive deal could be reached at that meeting.

Pompeo said the aim would be “an agreement ... such that the North Korean leadership will step away from its efforts to hold America at risk with nuclear weapons” and that Pyongyang should not expect rewards until it takes irreversible steps.

NORTH-SOUTH SUMMIT

News of Pompeo's trip came as South Korean President Moon Jae-in was preparing for his own summit with Kim, on April 27, with a bid to formally end the 1950-1953 Korean War a major factor.

"As one of the plans, we are looking at a possibility of shifting the Korean Peninsula's armistice to a peace regime," a top South Korean presidential official told reporters in Seoul.

"But that's not a matter than can be resolved between the two Koreas alone. It requires close consultations with other concerned nations, as well as North Korea."

South Korea and a U.S.-led U.N. force have remained technically at war with North Korea since the Korean War ended with a truce, not a peace treaty. The U.S.-led United Nations Command, Chinese forces and North Korea signed the 1953 armistice, to which South Korea is not a party.

The South Korean official said he did not know if any joint summit statement would include wording about ending the war, "but we certainly hope to be able to include an agreement to end hostile acts between the South and North."

Trump said on Tuesday he backed efforts between North and South Korea aimed at ending the state of war.

Such discussions between the two Koreas, and between North Korea and the United States, would have been unthinkable last year when North Korea conducted repeated missile tests, detonated its largest-ever nuclear bomb, and said the United States was now within range of its weapons.

The tests and escalating rhetoric between Trump and Kim raised fears of war before the North Korean leader called, in a New Year's speech, for lower military tensions and improved ties with South Korea.

Amid the diplomatic flurry, CNN reported that Chinese President Xi Jinping planned to visit Pyongyang soon, after Kim made a surprise trip last month to China, Pyongyang's sole major ally.

Despite China's traditional friendship with North Korea, Beijing has been angered by Kim's weapons development and backed successive rounds of U.N. economic sanctions, from which Pyongyang is seeking respite.On Wednesday, finance ministers from the Group of Seven industrialized countries issued a statement saying they were concerned about North Korea's evasion of sanctions and its "ability to access the international financial system."